1. Field of the Invention
This invention relates to sensing coils for fiber optic gyroscopes and to methods of manufacture thereof. More particularly, this invention pertains to improved quadrupole-wound fiber optic sensing coils and methods for their formation.
2. Description of the Prior Art
Fiber optic gyroscopes comprise two main components, (1) an interferometer (including a light source, beamsplitter, and detector) and (2) a fiber optic sensing coil. Light from the interferometer light source is split by the beamsplitter and applied to the ends of the sensing coil. The interferometer and associated electronics process the phase relationship between the two interfering, counter-propagating beams of light that emerge from (opposite ends of) the coil. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed.
It has been found that the effective operation of a fiber optic gyropscope requires a sensing coil of quadrupole-wound symmetry. This symmetry is attained by splitting the continuous fiber length into two source spools having equal lengths of fiber, and winding onto a reel, alternating source spools for each consecutive dual layer. A dual layer consists of two layers with their wrap starting and stopping at the same flange. The first layer is wound singly and forms the inner layer of the coil. Thereafter, layers are wound in alternating pairs from the two supply reels. Such an arrangement is shown in FIG. 1. As is seen, an optical fiber 10 is wound onto a take-up spool 12 from supply spools 14 and 16. The fiber 10 is wound from a single supply spool at a time and the other supply spool is rotated with the take-up or sensor spool to prevent the unwinding of previously formed layers. Thus, in FIG. 1, a layer of fiber 10 is wound onto the sensor spool 12 from the supply spool 14. The supply spool 16 is mounted for rotation with the take-up spool 12 by a common shaft 18. The supply spools 14 and 16 are alternately mounted to the shaft 18 to rotate with the sensor (take-up) spool 12 as the fiber 10 is fed onto the sensor spool 12 from the remaining supply or auxiliary spool.
The quadrupole winding pattern preserves symmetry about the center of the fiber and, as a result, decreases those phase errors that are otherwise induced by changing thermal conditions. The influence of changing thermal gradients upon phase errors, known as the Shupe Effect, is discussed, for example by N. J. Frigo in "Compensation of Linear Sources of Non-reciprocity in Signal Interferometers", SPIE Proceedings, Fiber Optic and Laser Sensors, Volume 412 (Arlington VA, Apr. 5 through 7, 1983) at pages 268-271.
The currently-recognized method for forming a sensor coil of quadrupole symmetry is illustrated by the set of drawing FIGS. 2A through 2C. In these figures, and the other figures of the application generally, the thickness of the fiber 10 is highly exaggerated to facilitate an understanding of the invention. A representative fiber optic sensor coil in accordance with the invention might comprise one thousand (1000) meters of 0.008 inch diameter fiber wound in ninety-two (92) layers onto a take-up spool having a one-half inch wide central core.
The conventional quadrupole winding process is begun by positioning the midpoint of the fiber 10 onto the central core 20 of the sensor spool 12 adjacent one of its opposed flanges 22 and 24.
Winding from the first of the supply spools along the direction 26, a first layer is formed atop the core 20 as shown in FIG. 2A. (Each cross-section of the fiber 10 indicates a turn of the coil winding. Turns wound from the two supply spools are distinguished by the presence -and absence- of interior stippling.)
After the initial layer is wound onto the sensor spool 12, the two supply spool leads 28, 30 are then positioned adjacent the flanges 22 and 24 as shown. The formation of this initial layer is considered part of the initial setup and is not performed again.
The relative positions of the supply spools (according to the arrangement of FIG. 1) are then rotated so that the layer formed in FIG. 2A is maintained and a second layer formed from the second supply spool by winding away from the "home" (left) flange 22 of the second supply to the flange 24 as illustrated in FIG. 2B. (The lead 30, at the same time, "pops up" as shown.) This is followed by a reversal in the direction of winding of the fiber 10 from the second supply spool to create a third (stippled) layer as shown in FIG. 2C.
The lead 30, as shown in FIGS. 2B and 2C, is upwardly directed to avoid "burial" under the stippled layers of the other supply and to permit the formation of a pair of (non-stippled) layers thereover from the first supply. The first of such layers, indicated at 32, is formed by winding from lead 30 in the direction 34 and the second, overlying layer 36 is formed by winding in the reverse direction 38. As noted, the lead 28 projects upwardly at the edges of these layers adjacent the flange 22 to permit the formation of a pair of layers from the second supply. The foregoing steps for forming pairs of layers are repeated, the supplies alternating every two layers as indicated by the contrasting turn markings of FIG. 2C, and the process continued until all of the fiber 10 is wound from the supply spools to the take-up or sensor spool 12.
The resulting coil has the property that lengths of fiber 10 that are equidistant from the center of the spool 12 are in close proximity and therefore of substantially identical temperature. As a consequence, temperature gradients are relatively symmetrical about the center of the wound coil. It follows from this that phase errors due to the Shupe Effect are likewise symmetrical about the center of the fiber and may, therefore, be cancelled.
A problem with the above-described technique for forming a quadrupole sensing coil resides in the fact that the indicated pop-up portions of the leads 28 and 30 are utilized to connect pairs of layers wound from the supply spools 16 and 14. When a supply spool is wound to its home flange, the other supply spool, from which the fiber is being wound, "pins" the lead to the flange, causing stresses (i.e. microbends) in the pinned lead and in the wrapped layers adjacent the pinned lead. Further, each lead pop-up causes distortions near the flange. Over several layers, such irregularities sum to distort the coil badly. In actual cases, the flat front of each layer can become so distorted that it is impossible to identify the layer and the winding pattern is lost. Due to the introduction of microbends, even coils that are successfully wound without losing their pattern can experience severely degraded optical performance. Further, such distortion of the winding pattern reduces the packing density of the coil which, in turn, can degrade and effectively limit the accuracy of the fiber optic gyroscope.
An important coil parameter that is significantly effected by microbends is the extinction ratio that characterizes the degree of isolation between the polarization modes of light transmitted through the wound cavity of a high precision fiber optic gyro. This parameter measures the ratio (in powers of dB) of the two polarization modes at the fiber exit port when all power is launched with one polarization at the entrance port. Optical fiber for forming a gyro is commonly provided in 1100 meter lengths characterized by a measured extinction ratio that exceeds 20 dB. It has been found that, when such fiber is wound or formed into a quadrupole configuration in accordance with the above-described conventional prior art method, the measured extinction ratio is degraded to less than 9 dB. This order-of-magnitude decrease in optical isolation represents a loss of signal energy and a diminution of instrument accuracy that is unacceptable for high performance gyros.